1. Field of the Invention
The present invention relates to a contact exposure apparatus suitable for forming a proof (color proof) in the field of printing process.
2. Description of the Prior Art
In the field of printing process, a large amount of film is used in a step of forming a printing plate consisting of halftone images from a continuous-tone image such as a reversal film, a color negative film or color paper, thereby separating colors and synthesizing an image. For this reason, the work procedures are complex to cause many failures such as character and layout errors. In addition, since a client irregularly requests changes in characters (e.g., prices), layout, colors or the like, an operator tends to forget to perform some of these changes. Also, whether a printed material has a desired color tone and gradation must be checked beforehand. All these checks are included in so-called "proofing".
For this purpose, in addition to a trial print called a "proof", a variety of systems are now commercially available. Examples are a CRT system, a system using a non-silver photosensitive material such as a photopolymer or diazo, a system in which an image is formed by a dry toner, and an electrophotography system. All these proofing systems, however, require a long time period to complete one color proof. In addition, a material cost is high and workability is poor. Therefore, the above systems are not satisfactory in practical use.
Another method of forming a color proof prints a color-separated halftone original on a color photosensitive material by color-separated light. For example, Japanese Unexamined Patent Publication (Kokai) No. 62-280746 discloses a method of obtaining a color proof on a color photosensitive material by a photographing system using a projection lens. Although an apparatus for use in such a method is large in size, it is designed to satisfy a so-called "roomlight specification" in which an operation can be performed in an ordinary room. That is, since a photosensitive material is kept inside the apparatus, exposure need not be performed in a darkroom but can be performed under normal illumination. Therefore, color proofing can be easily performed in a short time period at low cost to realize characteristics superior to those of the other proofing systems. Since the method utilizes the photographing system, however, an operation is adversely affected by flare or the like of a lens to degrade the gradation reproducibility. That is, by the flare or the like, a small-dot portion of a halftone image tends to be omitted, and its large-dot portions tend to become continuous.
Another method of forming a color proof from a color photosensitive material is a contact exposure system of an enlarge type such as "The Carlson Proofmaster System" (tradename) available from Chesley F. Carlson Company or a "Color Proofing System" (tradename) available from Kreonit, Inc. In these systems, a color-separated halftone film original is directly brought into contact with and exposed on a color photosensitive material. Therefore, the gradation reproducibility is largely improved as compared with that of the photographing system. The above systems, however, cannot be handled under roomlight because a color photosensitive material has a photosensitive range in the overall visible range. Therefore, an operator is forced to perform an operation in a darkroom. In addition, since a large light source device is necessary, an installation space of the apparatus must be increased and its operation is not easy. In this case, a compact light source may be used if a uniform slit beam is scanned to expose the entire surface so that the entire surface of an image forming region to be contact-exposed is uniformly exposed.
The present inventors proposed a color proof formation apparatus in U.S. Ser. No. 285,937. In this apparatus, a film original is overlaid on a photosensitive material, and they are set on an exposure table. Thereafter, a transparent cover sheet is covered on the film original, so that the film original is brought into contact with the photosensitive material through the cover sheet in vacuum. Exposure printing is then performed from the side of the cover sheet. This apparatus employs the contact exposure system, is compact, and allows roomlight operations.
In a scanning exposure apparatus such as a copier, parallelness and uniformity of light are not so important. In an apparatus for forming a color proof using a color-separated halftone original in the field of printing process, gradation reproducibility is an important problem. When a small-dot portion is omitted or large-dot portions become continuous, it is difficult to satisfactorily recognize reproducibility of the color-separated halftone original on a proof, and the reproducibility must be checked again using a film, resulting in a cumbersome operation. In addition, it is very difficult to recognize accurate image reproducibility and color reproducibility upon mixing of colors.
Furthermore, when a color-separated halftone original of a negative film is color-separation exposed on a negative color photosensitive material, one original film can be brought into contact with the photosensitive material. However, when a positive color-separated halftone original is color-separation exposed on a positive color photosensitive material, at least one of color plates (Y, M, and C plates), and a black plate, i.e., a total of two color-separated halftone films must be brought into contact with the photosensitive material. For this reason, one of the two color-separated halftone films is exposed on the photosensitive material through a base thickness corresponding to one film. Upon exposure of a positive color-separated halftone film for a printed material for which printing using a specific ink called "specific color" is simultaneously performed, two or more color-separated halftone films must be stacked and simultaneously exposed. In this system, parallelness and uniformity of light considerably influence reproducibility of an image. In particular, when parallelness is insufficient, satisfactory image reproducibility cannot be obtained even in contact exposure. More specifically, degradation of image quality such as formation of continuous dots, omission of dots, local fogging, or the like occurs due to dot fringes or the like. In particular, when a plurality of (two in FIG. 8A) originals are stacked and are directly exposed with diffused light from a light source, degraded reproducibility of the second original which is not in direct contact with a photosensitive material is observed. Of course, it is important to bring the color-separated original films and the photosensitive materials in tight contact with each other to keep flatness. For this purpose, in U.S. Ser. No. 285,937 described above, the interior of the covered region is subjected to vacuum suction to obtain flatness. However, even if flatness is satisfied, it is impossible to form a satisfactory color proof if collimated light cannot be obtained.